High frequency resonator and circuits therefor



July 22, 1947. p s, CARTER 2,424,267

HIGH FREQUENCY RESONATOR AND CIRCUITS THEREFOR Filed May 16, 1944 s Sheets-Sheet 1 I mmvron .--"L, PHILIP s. CARTER H W ATTORNEY E LOOP i July 22, 1947. s CARTER 2,424,267

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ATTORNEY July 22, 1947. s CARTER 2,424,267

HIGH FREQUENCY RESONATOR AND CIRCUfTS THEREFOR Filed May 16, 1944 6 Sheets-Sheet s Pk 0L A TE SP/IEkO/D OBLATE SPHERO/D Hove -GLA5$ INVENTOR PM; A 5. (14222-7? ATTORNEY v Patented July 22, 1947 HIGH FREQUENCY RESONATOR AND CIRCUITS THEREFOR Philip S. Carter, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 16, 1944, Serial No. 535,820

8 Claims.

This invention is a continuation-in-part of my application Serial No. 359,187, filed October 1,

1940, now U. S. Patent 2,357,313 granted September 5, 1944.

This invention relates to coupling circuits for passing a band of frequencies, and particularly to such circuits employing cavity resonators. The term cavity resonator is intended to include any high frequency electrical resonator comprising a closed electrically conducting surface enclosing a hollow space, and wherein the enclosure contains a periodically repeating electromagnetic field. The term coupling circuit used herein is intended to include any circuit which selectively passes a band of frequencies, such as, for example, an electrical wave filter, or a selective circuit, which might be used between stages of a receiver or transmitter.

In the communication field, it is often desirable to employ a four-terminal band pass coupling circuit which has two natural frequencies of oscillation differing by a small predetermined percentage of the mid frequency. Such fourterminal circuits may take the form of two coupled tuned circuits, one being connected. to the input terminals and the other to the output terminals, or may take the form of any suitable impedance network. It is known that such circuits may be made to obtain a band pass characteristic when loaded with a resistance. This resistance, which may constitute the useful load per se, serves to smooth out the double peak resonance curve of the four-terminal coupling circuit. It has been found, however, that when using ultra high frequencies it is impractical to construct such circuits of coils and condensers.

One of the objects of the present invention is to provide an improved coupling circuit capable of passing a desired band of frequencies with high efficiency, that is, with extremely low loss.

Another object is to provide a four-terminal coupling circuit which is compact and of extremely simple construction.

A further object is to provide a cavity resonator coupling circuit which has two natural frequen cies of oscillation differing by a predetermined percentage of the mid frequency and which possesses a desired band pass characteristic.

In the drawings:

Fig. 1 represents a square cavity resonator given for the purpose of exposition;

Figs. la and 1b illustrate respectively the electric field and magnetic field configurations of the resonator of Fig. 1;

Fig. 1c graphically illustrates the magnetic field and electric field distributions of the resonator of Fig. 1 as seen through a cross-section along line cc;

Figs. 1d, 1e and 1) illustrate three different magnetic field configurations which can exist in the resonator of Fig. 1, depending upon how the resonator is excited;

Fig. 2 illustrates a cavity resonator in accordance with one embodiment of the invention;

Figs. 2a and 2b illustrate, respectively, top and side views of the resonator of Fig. 2;

Figs. 3 and 3a illustrate the magnetic field configurations for two natural frequencies of the resonator of Fig. 2;

Figs, 4 and 4a graphically illustrate the resonant characteristics of the resonator of Fig. 2 plotted respectively as a function of frequency in megacycles per second and as a function of wavelengths measured in centimeters;

Fig. 5 shows an elliptic cylinder cavity resonator in accordance with another embodiment of the invention;

Figs. 5a, and 5b show the magnetic field configurations for the resonator of Fig. 5 when it is oscillating at the two desired modes of oscillation;

Fig. 6 graphically illustrates a system of confocal ellipses and parabolas for an elliptic resonator having certain dimensions;

Fig.7 shows certain dimensions of an elliptic cylinder resonator whose characteristics are illustrated in Fig. 71;;

Figs. 8, 8a, 9, 9a and 10 graphically illustrate various characteristics of an elliptic resonator in accordance with the invention under certain con ditions;

Fig. 11 is an elevation view, in section, and Fig. 110. a plan view of Fig. 11, of a circuit illustrating one way in which a cavity resonator of the invention may be used in association with an electron discharge device;

Fig. 12 illustrates the use of cavity resonators of the invention employed in a multi-stage system;

Fig. 12a is a plan view of the system of Fig. 12;

Figs. 13a, 13b and show the three projections of a prolate spheroid tank. Fig. 13a. isa side elevation, Fig. 13?) is a plan View, and Fig. 130 is an end view;

Figs. 14a, 14b and show the three projections of an oblate spheroid tank. Fig. 14a is a side elevation, Fig. 14b is a plan view, and Fig. 140 is' an end view; and 7 Figs. 15a, 15b and show the three projections of "an hour-glass and sphere typeof tank 3 where the two cones forming the hour-glass are made of elliptical rather than circular crosssection. Fig. 15a is a plan View, Fig. 15b is a side elevation, and Fig. 150 is an end View.

In order to aid in an understanding of the principles of the present invention, an exposition will now be given of certain oscillatory phenomenon leading up to the teachings of the present inven-. tion. Let us assume that we have a rectangular cavity resonator (sometimes called a tank) of square horizontal cross section whose sides are each 100 cm. in length, of the type shown in perspective in Fig. 1. In this cavity resonator the electric vector E is assumed to be vertical, as is shown more clearly in Fig. 1a, which figure illustrates the electric field configuration through a vertical section of the resonator of Fig. 1. The frequency corresponding to the fundamental mode of oscillation of the cavity resonator of Fig. 1 is approximately 212 megacycles corresponding to a wavelength of about 141.4 cm. The wavelength corresponding to the fundamental mode of oscillation is determined by the formula V2 multiplied by the length of one side of the square, in this case v'2 100. The height h of the cavity resonator of Fig. 1 is here considered to be immaterial, provided that it is sufficient to prevent breakdown of the dielectric within the resonator due to excessive voltage. It is assumed in this particular case that the dielectric is air, although it should be understood that other gaseous dielectrics may also be employed. The magnetic field configuration for the cavity resonator of Fig. 1 for the type of oscillation under consideration is shown in Fig. 1b, which represents a plan view of the resonator of Fig. 1. Fig. 1c graphically illustrates the magnetic field and the electric field distributions of the cavity resonator as seen through a cross section through the center of the resonator along the line cc. The electric intensity or electric vector, so to speak, is represented by the reference letter E, while the magnetic intensity or the magnetic vector is represented by the reference letter H. The current density along the top and bottom surfaces in the resonator is distributed in accordance with the H curve, while the voltagebetween the top and bottom of the cavity resonator is distributed in accordance with the E curve.

In addition to the fundamental mode of oscillation corresponding to a frequency of 212 megacycles, there will be found in the resonator of Fig. l other natural modes at all harmonics of this frequency. Moreover, there will also be found besides the harmonic mode a number of other modes whose frequencies are not in harmonic relation with the fundamental mode. For the particular cavity resonator of Fig. 1 having a square cross-sectional area, there is also a mode corresponding to a wavelength of 89.5 cm., and which has a natural frequency of 335 megacycles. This wavelength is obtained from the formula v gX 100 89.5 cm.

For this natural frequency of 335 megacycles, there are threemagnetic field configurations becomes which are shown in Figs. 1d, 1e and If, these figures indicating plan views of the cavity resonator of Fig. 1 and illustrating three ways in which the cavity resonator can oscillate at this particular frequency (335 mc.), depending upon how the resonator is excited. The dash lines 11, n and n in these last three diagrams indicate the nodal lines for the electric force within the cavity resonator. It should be understood at this time that the conducting square sides of the cavity resonator or the boundary, so to speak, sustain no electric force. The configuration of Fig. If is the resultant of a superpositioning of the two field configurations of Figs. 1d and 1e. Putting it in other words, the resultant magnetic field configuration of Fig. If is the equivalent of a cavity resonator of Fig. l oscillating at the same time in accordance with both of the magnetic field configurations of Figs. 1d and 1e.

In accordance with one embodiment of the present invention, the cavity resonator of Fig. 1 is modified somewhat from its square cross section, and made to have unequal length sides having dimensions a and b which may, for the sake of illustration, be assumed as cm. and cm., respectively. A modified cavity resonator in accordance with the invention is shown in perspective in Fig. 2. Figs. 2a and 21) show top and side views, respectively, of the resonator of Fig. 2. By exciting the resonator of Fig. 2 at or near one corner D by means of a loop K, extending within the interior of the resonator over a small distance and coupled to a suitable source of oscillations, the cavity resonator of Fig. 2 is made to have two natural frequencies of 335' megacycles :3%, rather than the single 335 megacycle mode which was obtained with the square cavity resonator of Fig. 1. By providing the cavity resonator of Fig. 2 with a suitable utilization circuit, constituted by way of example by the output loop M located at the diagonally opposite corner to the input loop K and loaded by means of a resistance R, the cavity resonator is made to have a band pass filter characteristic equivalent to that of a pair of conventional coupled circuits with the same natural frequency. Of course, there also exists a fundamental mode of oscillation of about 212 megacycles. and infinitely many higher order modes of oscillation.

Figs. 3 and 3a indicate the magnetic field configurations for the two natural frequencies of 335 megacycles L3%. The wavelength corresponding to these two natural frequencies of 0scillation, corresponding to the modes in which I am particularly interested here for the rectangular tank of Fig. 2, are determined by the following formulae:

in which M and A2 are the natural wavelengths, and a and b are the lengths of the two sides of the rectangular cavity resonator of Fig. 2. These formulae for A1 and we have been worked out from the solution of the partial differential equation of wave motion satisfying Maxwells equations, and employing rectangular coordinates because of the rectangular shape of the cavity resonator.

Fig. 4 graphically illustrates the resonant characteristics of the cavity resonator of Fig. 2, plotted as a function of frequency in megacycles per second. It should be observed that at a frequency of 335 megacycles minus 3%, there is a resonance peak indicated at point P which corresponds to the magnetic field configuration of Fig. 3, while at a frequency of 335 megacycles plus 3% there is another resonance peak P which corresponds to the other magnetic field configuration of Fig. 3a. It should also be noted that at around a mid frequency of 540 and a mid frequency of about 670 and also at a mid frequency of about 750 there are other pairs of resonant peaks which combine to give a band pass characteristic. At approximately 212 and 425 and 636 megacycles there are single resonance peaks corresponding to the fundamental mode of oscillation and its second and third harmonics, with which we are not concerned here.

Fig. 4a is somewhat like Fig. l, except that it graphically illustrates the resonance curves as a function of wavelength measured in centimeters. It should be noted that the mid wavelength of 89.5 centimeters corresponds to the mid frequency of 335 megacycles, shown in Fig. 4.

In proceeding to construct a tank circuit of the type shown in Fig. 2 in accordance with the present invention, the band of frequencies to be passed by the coupling circuit of the invention will, of course, be known. For most practical purposes, we may assume the band width of the cavity resonator filter or coupling circuit to be about 150% of the difference between the two natural frequencies. Hence, by substituting the two natural frequencie in the above mentioned formulae (1) and (2) for M and k2, we can obtain the dimensions of the cavity resonator of the invention,

The same general principles which have been discussed above apply to elliptic cylinder resonators which constitute another embodiment of the present invention, it being understood, of course, that different formulae apply to the elliptic resonator.

Fig. 5 shows a cylindrical cavity resonator having an ellipse as a cross section, in accordance with another embodiment of the present invention.

Figs. 5a and 5b show the magnetic field configurations for the elliptic cylinder tank of Fig. 5 when oscillating at the two mode of oscillation with which the present invention is particularly concerned.

hyperbolic cylinders.

In Fig. 5a the major axis of the ellipse is a voltage node, as indicated by the dash lines n3. In Fig. 5b the minor axis of the ellipse becomes the voltage nodal line 11A. The input and output coupling loops are located at opposite sides of the tank or cavity resonator at positions half way between the major and minor axes and are labeled K and M, respectively. The mean'or mid frequency In of the double mode of oscillation is given approximately by the formulae rcgqa corresponding to a wavelength of 0.814(1 centimeters. The exact determination of the two natural frequencies of the pair of modes ofoscillation is considerably involved, inasmuch as it incycles per second cludes the evaluation of Mathieu functions which are expressed as an infinite series of Bessel functions, but if A is the difference between the major diameter and the mean diameter of the ellipse, the two natural frequencies are approximately equal to f (1 :l: 2d

The precise mathematical equations necessary to obtain the exact dimensions of the ellipse will not be given here, in View of the extremely involved and complicated nature thereof. However, these exact dimensions for the elliptic tank of the present invention can be obtained from the solution of the partial differential equation of wave motion satisfying Maxwells equations and employing an orthogonal system of coordinates, wherein the coordinant surfaces are confocal, elliptic and The ordinary differential equations resulting from this method of procedure are of the so-called Mathieu type, as described in the book Modern Analysis by Whitaker and Watson, chapter XIX, published 1935, by Cambridge University Press, London. Graphs of the cyclic Mathieu function are shown in Jahne and Emde Tables of Functions, published 1938, by B. G. Teubner Leipszig 8: Berlin. Table of the Mathieu function of the radial type which are necessary in determining the exact dimensions of the elliptic cylinder have been worked out by the Physics Department of the Massachusetts Institute of Technology, Cambridge, Massa chusetts.

The laws governing the oscillation of an elliptic cylinder tank may be further explained by considering a resonator whose cross section is an ellipse having major and minor semi-axes of 110 and centimeters, respectively. This is equivalent to an eccentricity of 0.575. In order to obtain a solution of the partial differential equation of wave motion consistent with Maxwells laws and satisfying the boundary conditions of this problem, it is necessary to divide the space within the tank into an imaginary system of confocal, elliptic and hyperbolic cylinders. In this way we obtain an orthogonal system of coordinates and can separate the partial differential equation into two ordinary differential equations. We use the coordinates it, and Z as defined by the relations:

where a, is the semi-focal distance. I

The system of confocal ellipses and hyperbolas for an elliptic tank having dimensions cm. by 90 cm., and a semi-focal distance of 63.2 -cm., is shown in Fig. 6. The difierentialequations resulting from this process are of the Mathieu type as described in Whitaker and Watson Modern Analysis. The mathematical theory is quite in'-' volved and will not be discussed here. It'is necessary to find the natural frequencies by successive approximations. For this particular shape of elliptic tank the two natural frequencies corresponding to nodal lines on the minor and major axes respectively are shown in Fig. 7a. Fig. 7a.

also shows the natural wavelength for a circular tank having a radius equal to the minor axis of the ellipse, a similar tank having a radius equal to the major axis of the ellipse, and also one having a radius equal to the mean radius of the ellipse. It is seen that the mean of the two natural wavelengths of the elliptic cylinder is sub- 7 stan iall difi r nt than th natura a e eng h or a c r lar t k o he m e r d Consequently, the rule previously stated for determining the mean wavelength is seen to be only a very rough approximation. Fig. 7 shows the dimensions referred to in Fig. 7a.

In Fig. 8 is shown the distribution of voltage along the major axis of the ellipse when the tank is oscillating so that the minor axis is a nodal line. Fig. Ba shows the voltage distribution along the hyperbola designated by =45, when the tank is so oscillating that the minor axis is a nodal line. This curve serves to bring out the important fact concerning the oscillation of an elliptic tank that the focal points have no particular physical significance. It will be noted that the maximum voltage is not at a position corresponding to the focus and that nothing peculiar happens to the voltage distribution curve at the point corresponding to the focus. Figs. 9 and 9a show the voltage distributions along the major and minor axes, respectively, as a function of the distance from the center in centimeters, when an elliptic tank (110 cm. by 90 cm.) is oscillating at its fundamental mode. Although this mode is of no particular interest in connection with this invention, this voltage distribution curve is of interest in showing again that the focal positions have no particular significance with respect to the oscillation taking place.

Fig. 10 shows the way in which the natural frequencies vary with the eccentricity of an elliptic tank both for the fundamental and split modes. The natural wavelengths are given in terms of the major axis of the ellipse. The formulas for voltage distribution when the eccentricity is 0.575 are as follows:

First-For the fundamental mode c d a o e i co ne t on with Fiss- 2 and 5. may be used in connection with electron discharge devices. Although a rectangular tank or cavity resonator has been indicated in Fig. 11, the elliptical form can be used in identically the same manner.

Referring to Fig. 11, there is shown a rectangular cavity resonator I 0, constructed in accord ance with the invention, which is provided an aperture at a high voltage position of the res.- onator when it is oscillating at the mean frequency of the desired band, and which is penetrated by an inductive output tube H. The location of the aperture through which the inductive output tube H passes should be at approximately onequarter of the distance along the diagonal between the two corners I 2 and I3. If the resonator I0 is made to be an elliptical cylinder, the location is approximately the same, as can be seen from the voltage distribution curves previously discussed in connection with elliptical resonators. The elevation view, in section, of such an elliptical cylinder resonator, in association with an electron discharge device circuit for exciting the same, and an output circuit therefor, is identical with that shown in Fig. 11. The output loop M is located near the corner 43 opposite to the corner l2 nearest the inductive tube II, in order to provide working from a lower impedance point. The inductive output tube II is merely shown in its essential elements as comprising an electron envelope M containing therein a cathode li, a control grid 16, accelerator electrodes H, a collector electrode I8, and a magnetic field coil is. Suitable input energy is supplied from an input circuit 20, in turn coupled to a tuned circuit 2| connected between the cathode and grid. The control grid modulates the electron stream em- Second..Split mode with node on minor axis {1378-0392 cos 2+0.014 cos 4 i 4 E.= /g{1.2 J1(2.34 cosh r) +0.248J3(2.34 cosh 1,!/)+0.0l5J (2.34 cosh t Third-Split mode when node on major axis {1235 C05 O'248 cos 00s 5 E= (tanh {1.s30.r. (2.585 cosh 1,0)+O.935J3(2.585 cosh t) +0.101J5(2.5s cosh t) {1.830 sin 0.312 sin 34 +0.0202sin 5 i In these formulas (3), (4) and (5), J11 is the nth order Bessel function of the first kind, and the first bracket therein is an infinite series in which each term is a Bessel function of 211/); times the semi-focal distance multiplied by the hyperbolic cosine of the elliptic coordinate. This is the Mathieu function determining the variation of voltage with c. The coenicients in the terms of this series hold only for the particular ellipse assumed and are in general functions of wavelength and focal distance. The second bracket is a Fourier series and is the Mathieu function showing the variation of voltage with the coordinate 4). The coefficients in this series also depend upon the wavelength and semi-focal distance and arecorrect only for the particular dimensions assumed here.

Fig. 11 is an elevation view, in section, and Fig. 11a a plan view of Fig. 11, of a circuit illustrating a way in which either the rectangular or elliptical cavity resonator of the invention, de-

anating from the cathode when the input circuit 20 is excited. The passage of the modulated electron stream across the aperture of the res.-, onator or tank 10 induces a radio frequency cur-. rent in the resonator I 0, and since this tank circuit is tuned, a high voltage will be produced across the gap constituted by the aperture. The phase of this voltage at or near resonance will be such as to decelerate electrons traversing the gap during the one-half period of maximum intensity of electron current in the stream. These decelerated electrons are then collected at the low voltage collector l8. The kinetic energy lost by the electrons is transferred by the resonator or tank I0 into energy of the electro-magnetic field within the spaced angle by the tank circuit. This energy is thus transferred to the useful load by means of the output coupling loop M. The electronstream is focussed into a beam by combined efiects of the magnetic and electric fields. The magnetic field coil 19 is provided to aid in focussing the beam, while the accelerator electrodes accelerate and also serve to some extent to focus the electron beam. For a more complete description of inductive output electron discharge devices of this type, reference is made to an article by Dr. Haeif and Mr. Nergaard, entitled Wide band inductive output amplifier, published in the Proceedings of the I. R. E., March, 1940, and also to U. S. Patent 2,350,907 granted June 6, 1944, to F. H. Kroger and copending application Serial No. 527,278 filed March 20, 1944, by Fred H. Kroger.

FigsilZ illustrates the use of a cavity resonator in accordance with the invention, one as an interstage coupling device and another as an output device, in connection with a pair of inductive output type electron discharge devices. Fig. 12a. is a plan view of Fig. 12 showing more clearly the relative positions of the cavity resonators. Here again as in Fig. 11, although a rectangular cavity resonator has been shown, the elliptical cylinder form can be used in the same manner.

Referring to Fig. 12, there is shown in side elevation, a cavity resonator 22 used as an interstage coupling circuit between a pair of inductive output electron discharge devices ll, ll. One of these inductive output electron discharge devices ll serves as the input circuit for the output cavity resonator 23, in accordance with the invention. Inductive tubes II, II are located at high voltage point in the interstage resonator 22 which for a rectangular resonator are at onequarter and three-quarters of the distance along a diagonal. The location of the inductor tube I l in the output resonator 23 is also at a high volt age point, and the output loop M usually at a low voltage point. Of course, if the output circuit is to work into a high impedance, then the loop M may be located also at a high voltage point.

Although Figs. 11 and 12 have been shown in connection with electron discharge devices of the inductive output type, it should be distinctly understood that the resonators of the invention are not limited in their use solely to this type of electron discharge device, since they can also be used with the conventional type of vacuum tubes wherever there is need for the tuned circuits of applicants invention.

It will thus be seen that I have been able to construct a cavity resonator which has two natural frequencies of oscillations differing by a predetermined percentage and I have achieved this by departing from the symmetry of a square cross section, in accordance with one embodiment of the invention, and by departing from the symmetry of a circle in another embodiment of the invention.

Although the principles of the invention have been explained in connection with two specific forms of gravity resonators, such as the rectangular cross section and the elliptic cross section, it should be distinctly understood that the invention is not limited to these two forms but that other forms of cavity resonators may also be used which depart from the square and cross sectional resonators. Other forms of cavity resonators which may be used in accordance with the principle of the present invention are the hexagonal prism type with unequal length sides, the hourglass sphere type of resonator of the type described in my U. S. Patent 2,323,201 granted June 29, 1943, except that the cross section of the two cones is made elliptical rather than circular, and a modification of the hollow sphere to a prolate or oblate spheroid. The three projec- 10 tions of a prolate spheroid are hown in Figs. 13a, 13b and the three projections of an oblate spheroid tank are shown in Figs. 14a, 14b and 14c; and. the three projections of hour-glass sphere type of tank are shown in Figs. 15a, 15b and 15c. In these last three embodiments, the input and. output circuits have been shown as small dipoles instead of the loops shown in the other figures. These dipoles are interchangeable with loops and are illustrative of any suitable input or output circuits.

The resonator constituting the coupling circuit of the present invention may be used wherever a filter can be used and for substantially the same purpose, such as between stages of a receiver or a transmitter.

What is claimed is:

l. A cavity resonator comprising a hollow closed electrically conducting surface having a desired band pass characteristic, and means for exciting said resonator at such location that there are produced within said resonator two natural frequencies of oscillation difiering by a, predetermined percentage, the cavity resonator being in the form of a hollow prolate spheroid,

2. A cavity resonator comprising a hollow closed electrically conducting surface having a desired band pass characteristic, and means for exciting said resonator at such location that there are produced within said resonator two natural frequencies of oscillation differing by a predetermined percentage, the cavity resonator being in the form of a hollow oblate spheroid.

3. A cavity resonator comprising a hollow closed electrically conducting surface having a desired band pass characteristic, and means for exciting said resonator at such location that there are produced within said resonator two natural frequencies of oscillation differing by a predetermined percentage, said cavity resonator being in the form of an hour-glass sphere.

4. A cavity resonator comprising a hollow closed electrically conducting surface having a desired band pass characteristic, and means for excitin said resonator in its interior at such location that there are produced within said resonator two natural frequencies of oscillation which are relatively close to each other and differ by a predetermined percentage, the cavity resonator being hollow and comprised of a pair of elliptical cones arranged. along the same axis with their apices adjacent each other to simulate the general shape of an hour-glass, the bases of said cones terminating in a spherical conducting surface extending from one cone to the other.

5. A cavity resonator comprising a hollow closed electrically conducting surface in the form of a hollow prolate spheroid and having a desired band pass characteristic, and means for exciting said resonator in its interior at such a location as to produce therein two natural frequencies of oscillation which are relatively close to each other and dilfer by a predetermined percentage.

6. A band pass coupling circuit comprising a cavity resonator constituted by a hollow closed electrically conducting surface in the form of a hollow prolate spheroid and having a desired band pass characteristic, and means for exciting said resonator in its interior at such a location as to produce therein two natural frequencies of oscillation which are relatively close to each other and difier by a predetermined percentage, said means including a dipole, and an output circuit for said resonator comprising another dipole symmetrically arranged with respect to said first 11 dipole and 'on opposite sides 'of the center of said resonator. v

7;; A cavity resonator comprising a hollow closed electrically conducting surface in the form of a hollow oblate spheroid and having a desired band pass characteristic; and a probe exciting said resonator in its interior at such a location as? to produce therein two natural frequencies of oscillation which are relatively; close to each other and differ by a predetermined percentage.

8. A band 'pass coupling circuit comp-rising a cayity resonator constituted by a hollow closed electrically "conducting surface in the forrnof a hollow spheroid sa id resonator havin a band passjcharacteristie, element exciting said resonator in its interior at such a location as to produce therein two natural frequenciesof oscillation which are relatively close to each other PHILIP s. CARTER;

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Date Number Name 2,259,690 Hansen et a1. Oct. 21, 1941 2,227,372 Webster et ah Dec; 31, 1940 2,357,314 Carter Sept, 5, 1944 2,357,313 Carter Sent 5 

